A. Field of the Invention
The present invention relates generally to the field of inverter and switching device control systems and more particularly to a control system that utilizes an internal reference source and supervisory control circuitry to accurately control the operation of a switching inverter.
B. Description of the Prior Art
Switching inverters of various types and intended for various applications are well known in the prior art. For example, one common type of switching inverter is utilized for the generation of 110VAC/60HZ from a direct current voltage source such as a vehicle battery for situations where such power sources are not readily available. Other inverters are utilized for polyphase output generation.
For example, an inverter wherein the switching devices are controlled by an integrated circuit is disclosed in U.S. Pat. No. 3,921,005 which issued to D. L. Watrous on Nov. 18, 1975. The control circuit includes various stages for detecting and controlling operating parameters. Thus, a start-stop logic stage, a zero crossing detector, an AC high voltage inhibiting stage, a low DC battery voltage inhibiting stage, and a current set stage are provided.
U.S. Pat. No. 3,769,569 which issued to B. Doemen on Oct. 30, 1973, describes a polyphase inverter circuit utilizing clock generated pulses and digital circuitry for generating control signals for the switching device of each output phase.
Other inverter control circuits are disclosed in U.S. Pat. Nos. 3,913,000 which issued to G. I. Cardwell, Jr. on Oct. 14, 1975; 3,930,193 which issued to W. P. Kornrumpf et al. on Dec. 30, 1975; and 3,848,176 which issued to M. Etter on Nov. 12, 1974. These inverter control circuits vary the pulse width of the drive signals to the switching devices. For example, in U.S. Pat. No. 3,913,000, the pulse width of the drive signal is varied to equalize current to the switching devices. U.S. Pat. No. 3,930,193 describes the constant width pulse control of an SCR and the variable width pulse control of a second device in series with the SCR as determined by sensing the output power or current. U.S. Pat. No. 3,848,176 describes a chopper circuit supplying pulses of variable width to produce a variable output voltage of variable frequency under the control of an oscillator stage and digital control circuitry.
Another type of inverter control circuit of the prior art utilizes PWM (pulse width modulation) techniques to produce a sine wave output by the filtering of successive pulses of varying widths. Control circuits of this type are disclosed in U.S. Pat. Nos: 3,916,285 which issued to M. Iwata et al. on Oct. 28, 1975; 3,875,496 which issued to B. W. Carsten on Apr. 1, 1975; and 3,649,902 which issued to R. A. Dunbar on Mar. 14, 1972. The control circuit described in U.S. Pat. No. 3,916,285 utilizes a plurality of triangular waveforms to control different zones of operation defined by voltage and frequence in accordance with an input command signal. In U.S. Pat. No. 3,875,496, a high frequency triangular wave is compared with a corrected reference sine wave. The output of the comparator controls the switching devices. An over-current detector and an over and under voltage detector are also described. U.S. Pat. No. 3,649,902 is directed to a clock driven digital logic control system including divider stages and shift registers to control the switching devices in a PWM pattern to produce a sine wave.
While the above described inverter control circuits of the prior art are generally suitable for their intended use, the prior art inverter control circuits do not provide a control system utilizing an internal reference source and associated control circuit to establish accurately controlled start up and shut down of the inverter and timed control of the switching devices to establish desirable active/inactive duty cycles of each.